Rain made acidic by pollution can lower the pH of lakes.
Scientists measured the pH of the water in a lake over many years.
The table shows their results.
| Year | pH of lake water |
|---|---|
| 1960 | 6 |
| 1975 | 5 |
| 1990 | 4 |
| 2000 | 4 |
Describe how the concentration of hydrogen ions in the lake water changed between 1960 and 1990.
Include a calculation in your answer.
A student says:
"The lake water in 1990 was only 1.5 times as acidic as in 1960, because 6 ÷ 4 = 1.5."
Explain why the student is wrong.
In 2001, scientists added powdered calcium hydroxide, Ca(OH)₂, to the lake. The main acid in the lake water was nitric acid, HNO₃.
Write a balanced symbol equation, with state symbols, for the reaction between calcium hydroxide and nitric acid.
Write the ionic equation, with state symbols, for this neutralisation reaction.
A small pond nearby contains 2000 m³ of water with a pH of 4.
A scientist suggests raising the pH of the pond by adding pure water (pH 7) instead of calcium hydroxide.
Assume that the acidity behaves like an unbuffered strong-acid solution.
Calculate the volume of pure water that must be added to the pond to raise the pH to 5.
Explain why adding too much calcium hydroxide could harm the living things in the lake.
53 exam-style questions on OCR GCSE Chemistry 3.3 Types of chemical reactions, covering 3.3.1 Oxidation and reduction in terms of oxygen, 3.3.2 Acids, alkalis and neutralisation, 3.3.3 The pH scale and measuring pH, 3.3.4 Oxidation and reduction in terms of electrons, 3.3.5 Dilute and concentrated, weak and strong acids, and 3.3.6 Hydrogen ion concentration and pH. Each one has a worked solution and a mark scheme showing where the marks go.